1. Technical Field
The present invention relates to a nonvolatile memory element and to associated production methods and memory element arrangements and, in particular, to a nonvolatile memory element having a changeover material in which, after a forming step, at least two different conductivity states are realized and changeover between the conductivity states can be effected once or a number of times by the application of predetermined programming voltages.
2. Related Art
FIGS. 1A to 1C show a simplified sectional view and simplified U-I characteristic curves of a nonvolatile memory element of the generic type, as is disclosed in the document U.S. Pat. No. 5,360,981, for example.
In accordance with FIG. 1A, such a nonvolatile memory element has a first electrode 1, a changeover material 2 formed thereon and a second electrode 3, the electrodes 1 and 3 being correspondingly connected up for the application of a voltage and for the generation of an electric field E. The changeover material 2 comprises, for example, a hydrogen-saturated amorphous silicon semiconductor material (hydrogenated amorphous silicon) which has a p-type doping. By way of example, an electrically conductive material, and preferably Cr, is used for the first electrode 1. Suitable selection for the second electrode 3 results in either an analog changeover behavior for the changeover material 2 or digital changeover behavior. In accordance with FIG. 1A, by way of example, an analog changeover behavior is obtained with the use of V, Co, Ni and Tb, while a digital changeover behavior can be realized for the materials Cr, W, or Ag as second electrode 3.
What is characteristic of such nonvolatile memory elements is, in particular, a necessary forming step which is carried out at the outset and enables the actual nonvolatile memory properties of the memory element in the first place.
In accordance with FIG. 1B, by way of example, a linear U-I characteristic curve initially present is converted into a memory characteristic curve range in accordance with FIG. 1C only by the application of a forming voltage FA. Such forming voltages FA are relatively high voltages and usually lie in a range of 5 to 30 volts, in accordance with FIG. 1B a forming step being carried out at a forming voltage FA=−20 V.
Accordingly, it is only after this forming step has been carried out or after the application of this forming voltage FA in the changeover material 2 that a family KA of characteristic curves is generated which has nonvolatile memory properties and, by way of example, the two conductivity states or characteristic curve branches ON and OFF illustrated in FIG. 1C. In the case of the family KA of characteristic curves illustrated in FIG. 1C, Cr was used as electrode material and hydrogen-saturated amorphous silicon with a p-type doping was used as changeover material 2.
On the basis of this complex family KA of characteristic curves obtained after the forming step in accordance with FIG. 1C, it is then possible to realize an actual nonvolatile memory behavior, the conductivity states ON and OFF being traversed in the arrow direction by the application of corresponding operating voltages.
More precisely, by way of example, a changeover material 2 having the conductivity state ON can be reprogrammed by application of a programming voltage Verase of approximately 2.5 volts, as a result of which the conductivity state or characteristic curve branch ON switches to the further conductivity state or characteristic curve branch OFF. In the same way, the conductivity state ON can be generated again in the changeover material 2 by the application of a further programming voltage Vwrite of −3 V, for example. In this way, it is possible to switch back and forth between the two conductivity states ON and OFF in the family KA of characteristic curves or to effect programming, respective read voltages Vread being below the programming voltages and, in accordance with FIG. 1C, having 1 volt, by way of example. Since the family KA of characteristic curves or the conductivity states ON and OFF once programmed do not change in such changeover materials 2, a nonvolatile memory element is thus obtained with evaluation of an associated read current.
FIG. 2A shows a simplified sectional view of a further conventional nonvolatile memory element, in which case, however, the changeover material comprises a multilayer sequence. More precisely, by way of example, a p-doped hydrogen-saturated amorphous silicon 2A is formed on a first electrode 1, the surface of said silicon being adjoined by an n-doped hydrogen-saturated amorphous silicon layer 2B. Toward the second electrode 3, the changeover material 2 furthermore has an undoped, once again hydrogen-saturated amorphous silicon, as a result of which a so-called p-n-i structure is obtained. Although nonvolatile memory elements of this type have the advantage that the electrode materials are less critical in particular for p-doped semiconductor materials, the voltages for the necessary forming step are nevertheless even higher than in the case of the changeover material in accordance with FIG. 1A, which is why they have not been taken into consideration heretofore for mass production of nonvolatile memories.
FIG. 2B shows a simplified family KA of characteristic curves once again after a forming step has been carried out, resulting in an improved programming on account of the higher distance between the different conductivity states ON and OFF.
Therefore, the invention is based on the object of providing a nonvolatile memory element and associated production methods and memory element arrangements which can be used to realize an integration into conventional semiconductor circuits. In particular, the invention is based on the object of optimizing the forming step necessary for forming the nonvolatile memory behavior.